This is an excellent little tuner for guitars that don't have one. Install takes about 10 minutes and is done with double sided tape for easy removal. I mount the 9V battery on the back brace adjacent to the neck joint for the stealth factor. Neither the wires or the battery are visible from out in front of the guitar. This is a great little tuner with red flat/sharp indicators and a green in tune indicator. Accurate to within +/- 1 cent and bright enough to be seen outside. To answer the question, to my ear, it makes ZERO difference in tone or volume other than you can always make sure you are in tune.

The view from out front. All you can see is the tip of the toggle switch at about the 2 oclock position in the soundhole:

Unfortunately I have difficulty matching pitches by ear (i.e as in a tuning fork, piano, etc) which is why I'm terrible at tuning my guitars. Thats why I need one of those devices with lights that light up when the guitar is in tune.